When a delay of action occurs by the deflection of a machine, between the drive point and the machining point of a driven member to which the force from a drive source is applied, there arises a problem that an inconsistency or action occurs between the machining point, as commanded by a control system, and the actual machining point. This positional displacement due to the deflection is conspicuous in an industrial machine having a distance between the drive point and the machining point, such as a wire electric discharge machine.
Therefore, a representative example of the electric wire electric discharge machine is schematically shown in FIG. 1. The components of this wire electric discharge machine not described with reference numerals are omitted from FIG. 1.
A column 2, fixed on the bed 1 of the wire electric discharge machine, has UV axes and Z axis 3 fixed on its upper end and an arm for supporting a lower wire guide fixed on its intermediate portion. The arm extends through an arm cover 4 which is fixed on the column 2. On the bed 1, there is mounted a cross table 5, which can be moved in the directions of X- and Y-axes. On the cross table 5, there are mounted a work carriage and a machining bath 6 for dipping the workpiece. Between the upper wire guide, disposed at the leading end of the Z-axis, and the lower wire guide, disposed at the leading end of the arm, there is a wire electrode 7 extended for performing the electric discharge machining.
The arm, integrally fixed to the column 2, is extended passing through the arm cover 4 into the machining bath 6, and the workpiece, fixed on the workpiece carriage mounted on the machining bath 6 and the cross table 5, is movable relative to the arm, so that it is necessary to ensure that the machining liquid can be prevented from leaking at the joint between the arm and the machining bath 6 during the aforementioned relative movements.
The cross table 5, the machining bath 6 and the workpiece are moved in the X-Y plane by driving and controlling an X-axis driving servo motor Mx and a Y-axis driving servo motor My through servo control circuits 12 and 13 of a control system 11.
The wire electric discharge machine body is provided with a jet nozzle at the leading end of the Z-axis, a machining liquid circulator for supplying the machining liquid to the machining bath 6, various motors for feeding the wire, and a discharge machining power source for applying the machining voltage to the wire electrode 7. These equipments are individually driven and controlled by the control system 11 through a machining liquid control circuit 14, a wire control circuit 15 and a discharge control circuit 16.
The control system 11 is provided with a CPU 19 for the CNC to drive and control the individual axes of the discharge machining apparatus on the basis of the system program, as stored in a ROM 17, and a machining program, as stored in a RAM 18. The CNC CPU 19 is connected with the aforementioned servo control circuits 12 and 13 through a bus 20. The CNC CPU 19 outputs distributed pulses to the servo control circuits 12 and 13 for the individual axes to drive the servo motors of individual axes, move the cross table 5, and establish the discharge for the machining between the face of the workpiece, as carried on the workpiece carriage of the machining bath 6, and the wire electrode 7.
A CPU 21 for the PMC drives and controls the discharge control circuit 16, the wire control circuit 15 and the machining liquid control circuit 14 in accordance with the system program, stored in the ROM 17, and the power supply conditions and the machining conditions stored in the RAM 18.
The CNC CPU 19 is connected through a display control circuit 22 to a manual data input unit 23 with a display, so that the machining program and the power supply and machining conditions can be manually inputted. Furthermore, the machining route and the setting conditions can be monitored in the display screen of the manual data input unit 23.
With reference to the schematic diagram of FIG. 2, here will be described the detail of the joint portion between the arm cover 4 and the machining bath 6 in the electric wire electric discharge machine of FIG. 1. Incidentally, the components of the joint portion not denoted with reference numerals are omitted from FIG. 2.
On the back of the machining bath 6, there is mounted a seal plate 9, which is provided with a flange 8 for fixing the arm cover 4. This seal plate 9 is fitted to the back of the machining bath 6 by being pressed thereto with a number of bearings 10 so as to stop the hole, which is formed in the back of the machining bath 6 and extends in the X direction. A seal base is mounted on the outer circumference of the hole to prevent the leakage of the machining liquid from around the seal plate 9. On the inner circumference of the flange 8, there is fitted a V-packing for preventing the leakage of the machining liquid from around the arm extending therethrough.
As a result, when the cross table 5 is driven to move the machining bath 6 and the workpiece, especially when the machining bath 6 is moved in the X-axis direction, the seal plate 9, pressed to be fitted against the machining bath 6, will be dragged to move together with the machining bath 6 by the sliding resistance acting therebetween. Thus, a bending moment in the X-axis direction occurs to cause a deflection in the transmission mechanism, comprising the cross table 5 and the ball screws/nuts for driving the cross table 5, and in the joint between the transmission mechanism and the cross table 5 (hereinafter referred to together as the "workpiece drive unit"), so that the actual machining point delays from the commanded machining point. In this case,. the machining point is a position where the workpiece and the wire electrode 7 confront each other.
It is assumed, for example, that no external force in the X-axis direction is exerted upon the workpiece drive unit when the machining point commanded by the control system is located at (x, y)=(x1, y1), and that a command is given to move the wire electrode 7 from this state to the position (X, Y)=(x1+a, y1) (a&gt;0) (although the actual movement is made by the cross table 5).
In such a case, the aforementioned deflection occurs in the workpiece drive unit, causing the workpiece on the workpiece carriage of the cross table 5 to make a delayed movement. If the positional displacement due to this deflection is given as Ca' (Ca'&gt;0), the actual machining point is located at (x, y)=(x1+a-Ca', y1) so that the delay of Ca' occurs from the commanded machining point (x, y)=(x1+a, y1). The maximum A of this delay Ca' is dependent on the magnitude of the sliding resistance between the machining bath 6 and the seal plate 9 and by the rigidity and span of the individual members of the workpiece drive unit, and will not exceed the delay Ca' no matter how far the machining point is moved.
In other words, while the force to be applied in the +X direction from the machining bath 6 to the seal plate 9 is lower than the sliding resistance between the seal plate 9 and the machining bath 9, the seal plate 9 moves together with the machining oath 6 to increase the deflection Ca' of the workpiece drive unit in the -X direction. However, when the deflection of the workpiece drive unit increases until the reaction in the +X direction exceeds the sliding resistance between the seal plate 9 and the machining bath 6, a slip occurs between the seal plate 9 and the machining bath 6 to allow only the machining bath 6 to move. In short, the value of the deflection Ca' corresponding to the reaction, as acting in the +X direction when the slip occurs, takes the maximum value A.
As the correction method for correcting the positional displacement of a machine, there is known the pitch error correction for correcting the positional displacement caused by the pitch error of the feed screw, or the backlash correction for correcting the play of the power transmission mechanism occurring at the time of reversed movement of a driven member. However, neither of these methods can be applied to the correction of the positional displacement which is caused by the deflection.
More particularly, the pitch error correction determines the correction amount univocally according to the present position (i.e., the present position of the driven member with respect to the feed screw) of the driven member such as the cross table 5 in the mechanical coordinate system, so that it cannot cope with the mechanical deflection varying according to the moving distance after the reversal when the movement of the driven member is reversed at an indefinite position. On the other hand, the backlash correction for outputting, when reversing the movement of the driven member, the amount of correction for eliminating the play to be caused at the time of the change of the rotational direction of the gears or the like cannot cope with the deflection which gradually varies with the moving distance from the position at which the movement is reversed.
In short, there has never been proposed the correction method for eliminating the positional displacement caused by the deflection of the machine.